Shallow bulk acoustic wave devices

Abstract

Acoustic wave devices employing shallow bulk acoustic waves rather than surface acoustic waves, to provide higher frequencies of operation and other significant advantages, such as less susceptibility to aging and less sensitivity to surface contamination. In each embodiment of the invention, a transmitting transducer and a receiving transducer are oriented on the surface of an anisotropic piezoelectric crystal to achieve substantial coupling of shallow bulk acoustic waves and essentially zero coupling of surface acoustic waves. In one embodiment of the invention, various types of grating filters utilize shallow bulk acoustic waves, and a set of parallel mechanical or electrical discontinuities at or near the surface of the crystal provide reflection of acoustic energy at a frequency determined by the spacing of the discontinuities. Uniformly spaced gratings provide bandpass filter operation, and gratings with graduated spacings provide for frequency compression or expansion. In another embodiment of the invention, variations in device characteristics due to temperature changes are compensated for by directing the shallow bulk waves through materials having favorable or opposite temperature characteristics compared with those of the material into which the waves are originally launched. In other embodiments of the invention, a shallow bulk acoustic wave includes a multistrip coupler, for transferring electroacoustic energy from one propagation path to another, for use in bandpass filters, delay lines or unidirectional transducers.

We claim: 
     
       1. A shallow bulk acoustic wave device comprising: a block of anisotropic piezoelectric material having a surface, having physical properties which may be defined by reference to a set of orthogonal crystallographic axes, and being capable of propagating surface acoustic waves and shallow bulk acoustic waves; and   a pair of electroacoustic transducers disposed on said surface and so oriented with respect to said crystallographic axes that the electromechanical coupling coefficient for surface acoustic waves is substantially zero while the electromechanical coupling coefficient for shallow bulk acoustic waves is relatively large, whereby when an electric signal is applied to one of said transducers, a shallow bulk acoustic wave is launched in said block and propagated toward the other of said transducers, said other transducer serving to transform the shallow bulk acoustic wave into an output electric signal.   
     
     
       2. A device as recited in claim 1, wherein said material comprises rotated y-cut quartz. 
     
     
       3. A device as recited in claim 1, wherein said material comprises ST-cut quartz. 
     
     
       4. A device as defined in claim 1, wherein said transducers are disposed to launch a shallow bulk acoustic wave in a direction substantialy normal to the most desirable direction of propagation of a surface acoustic wave in the same piezoelectric material. 
     
     
       5. A device as defined in claim 1, wherein said material comprises double rotated quartz. 
     
     
       6. A device as defined in claim 1, and further including an external signal amplifier, and wherein the electrical signal applied to one of said transducers is derived from said amplifier, and the input signal to said amplifier is derived from said other transducer, whereby said device forms a feedback path for an oscillator circuit. 
     
     
       7. A shallow bulk acoustic wave device comprising: a block of anisotropic piezoelectric material having physical properties which may be defined by reference to a set of orthogonal crystallograhic axes, having a generally planar surface, and being capable of propagating surface acoustic waves and shallow bulk acoustic waves; and   first and second electroacoustic transducers disposed on said surface and being arranged in a predetermined configuration with respect to said crystallographic axes such that the electromechanical coupling coefficient for surface acoustic waves is negligibly small, said material serving to launch a shear wave in said block when an electric signal is applied to said first transducer, said shear wave propagating toward said second transducer as a shallow bulk acoustic wave, and said second transducer serving to transform said shallow bulk acoustic wave into an electric signal.   
     
     
       8. A shallow bulk acoustic wave device comprising: a pair of electroacoustic transducers;   means defining a path between said transducers for the transmission of shallow bulk acoustic waves, said means including a block of anisotropic piezoelectric crystalline material having a surface into which both surface acoustic waves and shallow bulk waves can be propagated, and said transducers being so oriented on said surface that the electromechanical coupling coefficient is practically zero for surface waves but is substantial for shallow bulk waves.   
     
     
       9. A shallow bulk acoustic wave device as set forth in claim 8, wherein said means defining a path between said transducers further includes coupling means for transmitting at least of portion of the energy associated with the shallow bulk acoustic waves from a first direction of propagation to a second direction of propagation. 
     
     
       10. A shallow bulk acoustic wave device comprising: a block of anisotropic piezoelectric material having physical properties that may be defined by reference to a set of crystallographic axes and having a surface into which both surface acoustic waves and shallow bulk acoustic waves may be propagated;   a sending interdigital transducer and a receiving interdigital transducer disposed on said surface and so oriented with respect to said crystallographic axes that coupling of surface acoustic waves is minimized and coupling of shallow bulk acoustic waves is substantial; and   a multistrip coupler disposed on said surface and having a portion of its width aligned with a first propagation path from said sending transducer and another portion of its width aligned with a second propagation path to said receiving transducer, whereby energy is coupled from said first propagation path to said second propagation path.   
     
     
       11. A shallow bulk acoustic wave device as set forth in claim 10, wherein: said transducers both have a weighted frequency characteristic;   said coupler comprises a plurality of straight, parallel metallized strips having part of their length normal to said first propagation path and another part of their length normal to said second propagation path, wherein substantially all of the energy received over said first propagation path is coupled to said second propagation path.   
     
     
       12. A shallow bulk acoustic wave device as set forth in claim 10, wherein: said second propagation path is parallel to and opposite in direction to said first propagation path;   said coupler includes   a set of parallel straight metallized strips having a first portion aligned with said first path and a second portion aligned with said second propagation path, wherein approximately half of the energy incident on said set of strips is coupled, and   two sets of U-shaped metallized strips which act as reflectors;   whereby wave components are of such a phase relationship after coupling that there is self-cancellation of components back along said first propagation direction, and all of the energy is transmitted along said second propagation direction.   
     
     
       13. A unidirectional transducer for a shallow bulk acoustic wave device, comprising: an interdigital transducer located on the surface of a block of anisotropic piezoelectric material at such an orientation that there is essentially no coupling of surface acoustic waves and substantial coupling of shallow bulk acoustic waves;   a U-shaped multistrip coupler disposed on said surface and having two parallel legs located one on each side of said interdigital transducer, with one of said legs being closer to said transducer by one-quarter wavelength of the shallow bulk waves transmitted by said transducer, whereby energy components are self-cancelling in one direction from said transducer, but are additive in the other direction.   
     
     
       14. A temperature-compensated shallow bulk acoustic wave device comprising: a block of anisotropic piezoelectric material having physical properties which may be defined by reference to a set of crystallographic axes and having a surface into which both surface acoustic waves and shallow bulk acoustic waves may be propagated;   a pair of electroacoustic transducers disposed on said surface and so oriented with respect to said crystallographic axes that coupling of surface acoustic waves is minimized and coupling of shallow bulk waves is substantial; and   temperature compensation means situated between said transducers, wherein said temperature compensation means includes a portion of a second piezoelectric material interposed in the path of the shallow bulk acoustic waves and having a temperature coefficient of velocity change less than or opposite to that of said block of piezoelectric material, whereby the overall temperature coefficient of velocity change is substantially reduced.   
     
     
       15. A device as set forth in claim 14, wherein: said block of piezoelectric material is comprised of two separate sections; and   said portion of a second piezoelectric material is located between said two sections and is bonded thereto.   
     
     
       16. A device as set forth in claim 14, wherein said portion of a second piezoelectric material is formed as a thin layer embedded in said surface between said transducers. 
     
     
       17. A shallow bulk acoustic wave device comprising: a pair of electroacoustic transducers; and   means defining a path between said transducers for the transmission of shallow bulk acoustic waves, said means including a block of anisotropic piezoelectric crystalline material having a surface into which both surface acoustic waves and shallow bulk waves can be propagated, and said transducers being so oriented on said surface that the electromechanical coupling coefficient is practically zero for surface waves but is substantial for shallow bulk waves;   and wherein said means defining a path between said transducers further includes at least one set of reflective elements located on said surface and so oriented with respect to the direction of propagation of the wave that the direction of propagation of selected frequency components is changed between said transducers.   
     
     
       18. A shallow bulk acoustic wave device as set forth in claim 17, wherein said reflective elements are uniformly spaced to provide reflection of acoustic energy within a relatively narrow band of frequencies. 
     
     
       19. A shallow bulk acoustic wave device as set forth in claim 17, wherein said reflective elements are nonuniformly spaced to provide different path lengths for different frequencies and therefore provide frequency compression or expansion. 
     
     
       20. A shallow bulk acoustic wave device comprising: a pair of electroacoustic transducers; and   means defining a path between said transducers for the transmission of shallow bulk acoustic waves, said means including a block of anisotropic piezoelectric crystalline material having a surface into which both surface acoustic waves and shallow bulk waves can be propagated, and said transducers being so oriented on said surface that the electromechanical coupling coefficient is practically zero for surface waves but is substantial for shallow bulk waves;   and wherein said means defining a path between said transducers further includes means for compensating for the effects of temperature on said device.   
     
     
       21. A shallow bulk acoustic wave device as set forth in claim 20, wherein said means for compensating for the effects of temperature includes two sets of reflective elements to reflect acoustic energy to a direction having associated with it a more favorable temperature coefficient of velocity change. 
     
     
       22. A shallow bulk acoustic wave device as set forth in claim 20, wherein said means for compensating for the effects of temperature includes a portion of a second piezoelectric material located intermediate said transducers, and having a more favorable or compensating temperature coefficient of velocity change as compared with that of said block of piezoelectric material. 
     
     
       23. A shallow bulk acoustic wave device comprising: a block of anisotropic piezoelectric material having crystallographic axes and having a surface into which surface acoustic waves and shallow bulk acoustic waves may be propagated;   a sending electroacoustic transducer and a receiving electroacoustic transducer disposed on said surface and so oriented with respect to the crystallographic axes that the electromechanical coupling coefficient is essentially zero for surface acoustic waves and is relatively large for shallow bulk acoustic waves, whereby shallow bulk acoustic waves can be launched into said piezoelectric material in a first propagation direction from said sending transducer;   a first set of reflective elements located at the surface of said piezoelectric material and inclined at an angle to said first propagation direction, said reflective elements being operative to change the direction of propagation of predetermined frequency components of the shallow bulk acoustic waves; and   a second set of reflective elements, operative to reflect predetermined frequency components of the reflected shallow bulk acoustic wave into a direction parallel with said first propagation direction, to be received by said receiving transducer; and   wherein said reflective elements are spaced nonuniformly to reflect different selected frequency components at different portions of said sets of reflective elements, whereby components at some frequencies traverse a longer path than components at other frequencies, and the times of occurrence of the various frequency components are compressed or expanded with respect to time.   
     
     
       24. A shallow bulk acoustic wave device as set forth in claim 23, wherein: the spacing between said reflective elements increases linearly from one side of said set of elements to the other side.